Many technologies have been introduced into vehicles to reduce their CO2 emissions, largely focusing on powertrain technologies and electrification. However, large reductions in emissions or energy consumption can be realised by altering driver behaviour. This work presents analysis of an onboard driver assistance system designed to encourage less aggressive driving through real-time audio and visual feedback, to examine its efficacy in Electric Vehicles (EVs). The device monitors aggressivity using the inertial power surrogate. Data is analysed from trials covering 36 900km of real-world driving in the U.K. using 14 diesel-powered vans. The reduction in energy consumption per kilometre that would be achieved using such a device in an EV is shown to be 4.1% on average, going from 568 to 544 Wh/km. Braking rates were also significantly reduced, which was unexpected since the eco-driving device did not directly promote this. This is significant because lowering the power level at which braking occurs allow a higher proportion of this energy to be recovered by the electric powertrain, especially for small batteries such as in hybrid or fuel cell EVs. Specifically, the maximum power requirement to capture 90% of kinetic energy was reduced by 12%, from 34kW to 30kW.